The commercial processing of photographic materials produces seasoned (used) Solutions containing silver ions. Environmental regulations restrict the discharge of solutions containing silver to concentrations much less than the silver concentrations generally found in seasoned solutions. Thus the silver concentrations in seasoned solutions must be greatly reduced before discharge into the environment.
A much improved precipitation process for removal of silver from seasoned silver containing solutions is disclosed in U.S. Pat. No. 5,288,728. A mercapto-s-triazine compound is used to precipitate silver. This process results in the formation of fine, sub-micron, colloidal particles of silver mercapto-s-triazine that prevent cost effective use of conventional separation methods.
One application of the mercapto-s-triazine precipitation method is the secondary, batch-mode recovery of silver by large photofinishers. This involves the batch recovery of silver from solutions that have already undergone another method of silver recovery to remove a major portion of the silver. Typically, the fine particles, referred to above, are allowed to settle to the bottom of the treatment vessel. Following a prolonged settling time, a sludge containing the fines is recovered after a decant operation and manually collected in filter bags. The lengthy settling time results in a prolonged total cycle time that necessitates installation of large and expensive equipment to provide the capacity to treat the volume of silver containing solutions generated over several hours.
A second potential application of this technique is the primary recovery of silver from solutions having concentrations of silver greater than 1 g/L. No method currently exists that provides efficient and economic phase separation because the large volumes of silver containing sludges generated.
U.S. Pat. No. 5.,288,728 suggests use of flocculants in combination with the precipitation process. However no directions are given concerning what materials might be used as flocculants. There are many different anionic, cationic and nonionic polymeric flocculants. A specific polymer may work well in some solutions to agglomerate precipitated fines. In other solutions that material may well cause a precipitate to disperse.